fanning the clutch helps you feed more power to the rear wheel when the engine starts to bog down. it helps when coming out of turns, coming up hills or any time your engine sounds like it might die.
when you pull in the clutch, the engine starts to rev and pretty much clears out the cylinder. you don't have to pull the lever in all the way, just enough to get the motor to rev. again, it's basically just used when the engine is under alot of load.
It's better to find the spot where the clutch is slipping enough to keep the RPMs up, but still pulling hard. Fanning it in and out is a bad idea as you're only at the optimum point briefly as you alternate between slipping the clutch too much and not slipping it enough to keep the RPMs up.
The smaller the bike, the harder it is to find and keep that spot.